Best practices for improving form performance? (Planning)

Summary:

What are some best practices for improving form performance?

Content (required):

I've got an input form for updating forecasts on a monthly basis. There is a business requirement to also include a summary by Opex category so the end user can see their changes on the fly at an aggregate level. So, I have a section at the top of the form for input, with a list of valid Accounts for updating, and a summary by Opex category below this.

Data submission on this form takes up to 45 seconds, which I believe will be unacceptable for the business.

I have a business rule calculation which runs upon save, but this is not the cause of the slow form performance; the rule only takes 6 seconds to run, and I've tried removing the rule from the form, and it's still slow.